1. Field of the Invention
The present invention relates to a positively chargeable toner for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing, etc. and a charge control agent for positive charging capable of controlling the amount of charges of the toner.
2. Description of the Prior Art
In copying machines etc. based on electrophotography, various dry or wet toners containing a coloring agent, a fixing resin and other substances are used to visualize the electrostatic latent image formed on the photoreceptor having a light-sensitive layer containing an inorganic or organic photoconductive substance.
The chargeability of such toners is one of the most important factors in electrostatic latent image-developing systems. Thus, to appropriately control the amount of charges of the toner, a charge control agent providing a positive or negative charge is often added to the toner.
Examples of conventional charge control agents providing a negative charge for toners include the chromium complex salts, cobalt complex salts and iron complex salts of azo dyes, which have a relatively good charging property. Examples of conventional charge control agents providing a positive charge for toners include the nigrosine dyes disclosed in Japanese Patent Examined Publication No. 2427/1966 and other publications. However, many of these charge control agents are relatively dense colored.
For the charge control agent to be generally applicable to color toners of various colors, it is required to be colorless or light-colored to such extent that it does not adversely affect the tones of the color toners. Charge control agents for negative charging known to meet this requirement include chromium complex salts, zinc complex salts, aluminum complex salts and boron complex salts of aromatic hydroxycarboxylic acids such as salicylic acid and alkylsalicylic acid or aromatic dicarboxylic acids; silicon derivatives of mono- or poly-cyclic diols; and calix(n)arene compounds. On the other hand, charge control agents for positive charging known to meet the above-mentioned requirement include quaternary ammonium salt compounds such as those described in Japanese Patent Examined Publication Nos. 36938/1989, 57341/1989, 54696/1989 and 20905/1992, pyridinium salt compounds such as those described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 87974/1987 and 98742/1983, and polyamine resins such as those described in Japanese Patent Examined Publication No. 13284/1978.
However, conventional charge control agents for positive charging are insufficient in compatibility for toner resin and triboelectric charging effect in some cases and cannot contribute to charge stability during multiple repeated use of the toner in other cases. In addition, initial copied images are relatively low in sharpness or the quality of copied images during continuous copying is relatively changeable because the charging rise speed is insufficient. For these reasons, there is a demand for the development of a charge control agent for positive charging free of the above problems to be solved for charge control agents for color toners.
The object of the present invention is to provide a charge control agent for positive charging which is versatile for use in various color toners, including the three subtractive primaries yellow, magenta and cyan colors, and achromatic toners, which is excellent in charge control property stability to changes in temperature and humidity, i.e., environmental resistance, charge control property stability over time, i.e., storage stability, and charge control property stability during multiple repeated use of toner, i.e., durability, and which offers rapid toner charging rise, and a positively chargeable toner for developing electrostatic images which can be used as various chromatic or achromatic toners, which is excellent in environmental resistance, storage stability and durability and which offers rapid charging rise.